The importance of T-cell immunity in murine malaria was confirmed by demonstrating increased susceptibility of adult thymectomized, x- irradiated, bone marrow reconstituted (ATX-BM-ATS) BLCF, mice and congenitally athymic (nude) NIH and BALB/c mice. An anti-theta sensitive in vitro for T-cell immunity in murine malaria was developed, measuring H3-thymidine incorporation of lymphocytes cultured in the presence of parasitized blood or a saline soluble antigen prepared from ficoll gradient purified schizonts by successive freezing and thawing. With this assay, a T-cell response can be followed through the course of a non-lethal P. berghei infection, and attempts can be made to characterize those antigens which stimulate T-cells in vitro.